This invention pertains generally to methods for converting analog signals into digital form and more particularly to analog to digital conversion methods that maximize noise rejection.
In many applications analog electrical signals are monitored as an indication of events occurring at remote locations. Frequently, it is necessary to transport the analog signals through adverse environments which contribute noise to the signal being conveyed. Often the noise contributions will obscure the information being communicated. While a number of noise rejection techniques are available in the form of filters, the slow response times exhibited are often objectionable. Noise problems become even more acute when further processing of the analog signals is required to obtain meaningful data. Arithmetic computations on the communicated information will in certain cases amplify the effective noise to signal ratio. In a number of applications, such computations are more efficiently obtained by first providing a digital representation of the analog input. However, in a high noise environment the digital samples will be severely affected by superimposed noise components.
Noise rejection problems as well as the problems associated with converting analog signals to digital form become even more acute in many industrial systems in which minicomputers are employed to interface with analog signals remotely generated. An example of such a system is the axial power distribution monitoring system employed as part of a number of pressurized water reactor surveillance systems. An example of such a system is described in U.S. Pat. No. 3,932,211 entitled "A Method of Automatically Monitoring the Power Distribution of a Nuclear Reactor Employing Movable Incore Detectors", by James J. Loving, Jr. issued Jan. 13, 1976. The purpose of the system is to periodically scan the reactor core using movable incore flux mapping detectors. The neutron flux throughout the axial height of the core is recorded, normalized, and searched for unusual peaks that exceed acceptable limits. Unusual peaks in axial flux generally indicate abnormalities in the core such as gaps between fuel pellets caused by densification. The localized power increases that result must be kept within acceptable limits to insure the effectiveness of emergency core cooling systems in the unlikely event of severe accident conditions.
For maximum efficiency, it is necessary to compare normalized data, such as neutron flux divided by the average over the core height, to a variable threshold. Acceptable peaks are then determined as a function of axial position. Higher peaks can be tolerated in the bottom of the core than can be tolerated at the top of the core. The alarm threshold is therefore, monotonically decreasing with increasing height in the core. To perform this function properly, the raw data must be sampled and stored throughout a scan since the true average can only be calculated at the end of each scan. A normalized curve must be generated and compared to a variable alarm threshold. An analog implementation of this function would be highly expensive and complex compared to a digital approach with a large number of samples. To accomplish this result it is desirable to employ a bus oriented minicomputer system. However, data conversion and transfer is complicated by the severe electrical environment experienced under the ambient conditions associated with nuclear reactor facilities.
The axial power distribution monitoring system, like many other systems that employ digital minicomputers, requires that all inputs and outputs be interfaced by input/output circuitry located outside the computer. In addition, the analog signals must be converted to a digital representation by the input/output circuitry before being communicated to the computer. While the internals of the minicomputer are free of electromagnetic inteference due to appropriate shielding and filtering techniques, the input/output electronics experience a more severe environment since the remainder of the system employs very little shielding or filtering. Inexpensive successive approximation analog to digital converters used generally on input/output cards are particularly sensitive to interference on the incoming analog signals. Even when the analog signals are sufficiently processed to cleanse them of interference, and/or dual slope analog to digital converters are employed, the logic and wiring from the converter to the computer remains susceptible to interference either from the power supply lines or radiation from other signal lines.
Accordingly, in the axial power distribution monitoring system, as in many systems, the need exists for a simple, inexpensive technique to accept low speed analog signals, and convert and transfer them within an electrically noisy environment to a separate minicomputer. This must be accomplished with a minimum susceptibility to electromagnetic interference and without expensive shielding and filtering of the entire system.